Hide ARM assembly memset symbols.

Prevent these symbols from being exported from any library which
includes skia by marking them .hidden, as they are implementation
details of skia.

BUG=skia:3303

Review URL: https://codereview.chromium.org/831663004
diff --git a/src/opts/memset.arm.S b/src/opts/memset.arm.S
index 44b75e3..8c82f74 100644
--- a/src/opts/memset.arm.S
+++ b/src/opts/memset.arm.S
@@ -18,8 +18,10 @@
     .syntax unified
 
     .global arm_memset32
+    .hidden arm_memset32
     .type   arm_memset32, %function
     .global arm_memset16
+    .hidden arm_memset16
     .type   arm_memset16, %function
 
 /*
diff --git a/src/opts/memset16_neon.S b/src/opts/memset16_neon.S
index 7b2b190..441b523 100644
--- a/src/opts/memset16_neon.S
+++ b/src/opts/memset16_neon.S
@@ -21,6 +21,7 @@
         .fpu neon
         .align 4
         .globl memset16_neon
+        .hidden memset16_neon
 
 memset16_neon:
         cmp             r2, #0
diff --git a/src/opts/memset32_neon.S b/src/opts/memset32_neon.S
index fa03476..993084a 100644
--- a/src/opts/memset32_neon.S
+++ b/src/opts/memset32_neon.S
@@ -9,6 +9,7 @@
 	.fpu neon
 	.align 4
 	.globl	memset32_neon
+	.hidden memset32_neon
 
 	/* r0 = buffer, r1 = value, r2 = times to write */
 memset32_neon: